The structure of the polyols obtained according to this method is known, but it has never been possible to obtain this structure industrially in an economical and practical manner. This structure is formed by the linkage of ##STR1## units in longer or shorter sequences, depending on the lactone/epoxide ratio, with the ends of the molecular chain being composed of hydroxylated polyether sequences.
The preparation of copolymers from lactones and epoxides is also known, but the copolymerization reaction takes place in an acid medium, more particularly in the presence of a Lewis acid. In that case, the lactone is the least active one of the monomers; the epoxide tends to react first so as to form polyether sequences and then the lactone reacts so as to form polyester sequences interspersed with ether units when the reaction velocities establish an equilibrium as a function of the concentrations of each of the monomers.
As a result, the structure of the copolymers presents itself in the form of a chain with central polyether sequences and with ends rich in polylactone sequences according to the schematic formula: EQU I--POA--POA--PCL--.sub.x OH].sub.y
in which I represents the remainder of the initiator, POA represents the polyether sequence and PCL represents the lactone sequence.
Such a structure leads to polyols, generally solid polyols at ambient temperature no matter what the molar ratio of lactone to epoxide is, which confer upon the polyurethanes a relationship of properties to ester content which is not very advantageous as compared to classic polyether or polyester polyols in the form of homopolymers.
In U.S. Pat. No. 2,962,524 and French Pat. No. 2,240,248 procedures are described which make it possible to improve the statistical distribution of the oxylactone and oxyalkylene units noted above without, however, modifying the structure of the copolymers as shown in the schematic formula above.
These procedures are still based on the method of direct copolymerization by acid catalysis. In these procedures, the difference in reactivity between the lactone and the epoxide is compensated for either by delaying the introduction of the more reactive monomer into the copolymerization medium, with the reagents being introduced separately, or by introducing the lactone and the epoxide as a mixture at a rate below the intrinsic reaction velocity of the least reactive of the monomers in such a way as to avoid its accumulation in the reaction medium. Since any modification of the reactivity brings with it an important modification of the structure of the polyols and because of the highly exothermal character of the addition reactions of the lactone and the epoxide, it is indispensable that the reaction must be carried out at a relatively low temperature, of the order of 70.degree. C., in order to obtain satisfactory amounts and liquid polyols in a reproducible manner. This latter temperature limitation results in a low productivity for the procedures and makes it necessary to carefully control the temperature of the medium and the concentration of the monomers during the reaction of synthesis, and this becomes expressed in relatively high manufacturing costs.
Until the appearance of the method which is the object of this invention, in spite of the imperfections as far as the process itself and its results are concerned, only the copolymerization by acid catalysis permitted the manufacture of polyols (by the copolymerization of lactones and epoxides) which were likely to be suitable for the synthesis of polyurethanes. The systematic use of acid catalysts or coordinated organometallic catalysts of the boron trifluoride or tetrabutyl titanate type probably is due to the fact that it has always been taken for granted that the preparation of polyester polyols that are homopolymers based on lactone is impossible by basic catalysis since under these conditions the homopolymerization of the lactones is described as a slow reaction leading to low conversion rates of the lactones and to bad yields. Statements to this effect can be found in: MAKROMOL. CHEM. 1968, 113, pages 139-146, "Anionic Copolymerization of Lactones in Correlation with the Mode of Fission," and MAKROMOL. CHEM. 1972, 152, pages 291-303, "Synthesis and Characterization of Several Poly-.epsilon.-caprolactones."